1. FIELD OF THE INVENTION
The invention relates to a unidirectional space division transmission system employing four-layer semiconductor switching elements (for example, PNPN transistors or PNPN diodes) for a cross-point switch.
2. DESCRIPTION OF THE PRIOR ART
As well known, a four-wire channel is generally used in the transmission system for video signal exchange in video phones in order to facilitate insertion of repeater amplifiers in the transmission system. Thus, the four-wire channel transmission system may be considered in construction as a combination of two systems each having unidirectional two-wire channel. This will be described in more detail with reference to FIG. 1.
In FIG. 1 illustrating in block form such a transmission system for video signal exchange, SUB A and SUB B designate video telephone sets, NW.sub.1 and NW.sub.2 networks for video signal exchange, CTL a control unit for controlling the networks NW.sub.1 and NW.sub.2, 1, 2, 9, and 10 subscriber lines, 3, 4, 7, 8, 11, and 12 links, and 5 and 6 junctors. Control for connection between calling and called subscribers is carried out by a voice signal exchange system not shown. The video signal derived from the video telephone set SUB A is transmitted to the video telephone set SUB B through the channel of subscriber line 1 -- link 3-- junctor 5-- link 7-- subscriber line 9, while the video signal derived from the video telephone set SUB B is transmitted to the video telephone set SUB A through the channel of subscriber line 2-- link 4-- junctor 6-- link 8-- subscriber line 10. From the just-mentioned construction of the channels, it will be seen that the signals pass always in the same direction through the network NW.sub.1 or the network NW.sub.2, and that a pair of two-wire channels are independently established through up links 3 and 7 and down links 4 and 8.
In FIG. 2 is shown a conventional transmission system in which each of the channels in FIG. 1 is comprised of four-layer semiconductor switching elements. In this instance, PNPN transistors (hereinafter referred simply to as thyristor) are employed as the four-layer semiconductor switching elements. In the figure, NW designates a network comprising thyristors, IB and input connection circuit located for connection between the subscriber line and the network NW, OB an output connection circuit located for connection between the network NW and another subscriber line, CS a current control circuit for controlling the dc bias current so as to hold the thyristors conductive and CTL is a control unit.
More particularly, in FIG. 2, +VH designates a power source for the dc bias current, +VB another power source for a bias supply, 1 to 4 thyristors which cooperatively form a cross-point switch means generally designated by 50, 5 to 8 resistances each for setting a cathode potential of each of the thyristors when they are non-conductive, 9 a current restricting resistance, 10 and 11 choke coils, 12 an input transformer 13 and 14 capacitances for blocking dc current flow. Means generally designates by 52 is provided for control the dc bias current and includes constant current circuits 15 and 16, a diode 17 for preventing a reverse-bias application, and memory devices 18 and 19 for controlling the constant current 15 and 16. The output connection circuit OB includes an output transformer 20, capacitances 21 and 22 for blocking dc current flow, an output transformer 20, and an amplifier 23 to compensate for the insertion loss due to the use of the thyristors 1 to 4, the input transformer 12, the capacitances 13, 14, 21 and 22; the choke coils 10 and 11, and the output transformer 20. Switch means 51 and control means 53 which are similar to the switch means 50 and control means 52 as above-mentioned, constitute another route parallel to the route including the means 50 and 52.
The operation of the transmission system in FIG. 2 will be described below. A voice signal control unit (not shown) first issues a commond requesting a formation of a channel for video signal. In response to the command, the control unit CTL acts to select the corresponding input and output connection circuits IB and OB, and deliver necessary control signals to the thyristors in the network NW and to the current control circuit CS. Thereby to establish a channel for the video signal as requested. More precisely, the control unit CTL delivers a control signal for activiating the memory devices 18 and 19 associated with the corresponding constant current circuits 15 and 16. Upon receipt of the control signal, the memory devices are set to drive the associated constant current circuits 15 and 16. Following this, the control unit CTL applies gate pulses to the gates of the thyristors 1 to 4 so as to fire these thyristors in the order of the one closest to the circuit CS, next closer one and so on, i.e. 4, 3, 2 and 1. When the thyristor 1 is fired, the dc bias current flows through two routes of power source +VH -- resistance 9 -- choke coil 10 -- thyristors 1, 2, 3 and 4 -- constant current circuit 15 -- ground, and +VH -- resistance 9 -- choke coil 10 -- thyristors 1, 2, 3 and 4 -- diode 17 -- constant current circuit 16 -- ground. The dc bias current is determined in value by the constant current circuits 15 and 16. Similarly, the route of power source +VH -- resistance 9 -- choke coil 11 -- switch means 51 -- control means 53 -- ground serves to supply the switch means 51 with a DC bias current. If the dc bias current is above the holding current of the thyristors 1 to 4, these thyristors 1 to 4 remains conductive even after the gate pulse is removed. In this manner, a channel is completed through ac coupling between the inputs a.sub.1 and b.sub.1 and the outputs a.sub.2 and b.sub.2. The channel is interrupted by resetting the memory devices 18 and 19. That is, the reset of the memory devices 18 and 19 causes the constant current circuits 15 and 16 to be interrupted. As a result, the dc bias current reduces to zero and thus thyristors 1 to 4 turn off by themselves.
An explanation will be given about the operation of the diode 17 for preventing a reverse bias application. Assume now that the constant current circuit 15 and the diode 17 are eliminated from the circuit of FIG. 2. If the thyristors of the switching means 51 are fired before firing of the thyristors 1 to 4, a current flows through the route of power source +VH -- resistance 9 -- choke coil 11 -- switch means 51 -- control means 53 -- ground. This current flow causes the voltage level at the junction between the control means 53 and the capacitance 22, and thereby the terminal voltage V of the constant current to rise. The rise of the terminal voltage also would increase the cathode potential of the thyristor 4 which in turn would block the gate current flow from the gate thereby to prevent the thyristors 1 to 4 from being fired. For avoiding such firing trouble of the thyristors, an arrangement is needed in which the diode 17 is provided between the constant current circuits 15 and 16. The control of the channel in such a manner by using thyristors is well known. Incidentally, it might be considered that inductances of the coils used in the input and output transformers 12 and 20 could be utilized for arranging the dc bias current supply circuit mentioned above. But, practically the choke coils 10 and 11 and the constant current circuits 15 and 16 are additionally connected for the purpose. This is because it is hard to obtain a transformer which is applicable to a broad range of frequencies with dc bias superposed thereon, from an economical standpoint.
The conventional transmission system so far described uses transformers and capacitors in the input and output circuits, resulting in the following especially, when it is used for a transmission of video signals in a broad frequency band from several tens of Hz to several MHz.
That is, the input transformer 12, the output transformer 20, the choke coils 10 and 11 must be large in size because it is required to pass signals of low frequencies of several tens of Hz without distortion. This prevents the practical application of the conventional transmission system on fitting and economical grounds.
Further, the frequency characteristic of the transmission loss in the channel is hardly improved because of the use of the input transformer 12 and the output transformer 20. For this, both of the transmission loss and variation of the loss are large either in low frequency range below 100 Hz or in high frequency range above several hundreds of KHz. Accordingly, the conventional transmission system could not achieve satisfactory performance, such as satisfying the quality standard in transmission.
Moreover, two constant current circuits are needed for the current control circuit CS so that the system becomes large in size, and additionally the constant current circuit necessitates some means to radiate heat, and thus the system becomes expensive.
Further disadvantages are that if the current path including the thyristors 1 to 4, and the current control circuit CS is erroneously short-circuited to the ground during a test of adjustment, an earth current flows through a path of, for example, power source +VH --resistance 9 -- choke coil 10 -- thyristor 1 to the ground. The earth current is restricted somehow by the resistance 9 and the choke coil 10. However, it is difficult to reduce the earth current below the current capacity of the thyristor when considering the voltage withstand and the current capacity of the thyristor, and the value of dc bias current. Therefore, whenever the earth current flows, the thyristor is generally damaged.